In the integrated circuit (IC) industry, chemical mechanical polishing (CMP) is being used to polish both metallic and dielectric layers. In order to chemically mechanically polish (CMP) a wafer, a platen/table is provided where a polishing pad is placed on top, of the platen. A chemical slurry is then dispensed on the polishing pad and the polishing pad is rotated. While the polishing pad is spinning, a semiconductor wafer, which is attached to a wafer carrier, is brought in contact with the slurry-covered wafer pad. A chemical and mechanical interaction between the slurry, the polishing pad, and a top surface of the wafer results in material being removed from a top surface of the wafer whereby a planar top surface topography of the wafer is achieved after a certain polishing time period.
A problem confronted by chemical mechanical process engineers is the fact that the polish rate of the center of the semiconductor wafer is much slower or different from the polish rate at the edge of the wafer. These different polishing rates, which are due to lack of uniform slurry transport, rotational speed variations across the radial surface of the wafer, etc., result in the polished surface of the wafer being substantially non-uniform. This non-uniformity CMP problem is clearly illustrated in prior art FIG. 1. FIG. 1 illustrates a semiconductor wafer 10. A top surface of the semiconductor wafer 10 has been polished in FIG. 1. FIG. 1 illustrates a central portion 12 of the wafer 10 and an edge portion 14 of the wafer 10. As can be clearly seen from FIG. 1, the central portion 12 of the wafer 10 has been polished at a much slower rate than the edge portion of the wafer 14 resulting in a thicker central portion of the wafer 10. The difference in wafer-scale material thickness after completion of chemical mechanical polish processing results in reduced yield due to either over-polishing at the edge 14 or under-polishing at the center 12.
The integrated circuit (IC) industry has tried many different methods in an attempt to reduce the non-uniform polishing surface illustrated in FIG. 1. A first method has attempted to make polishing pads that have varying pore densities across a surface of the pad. In one form, a pore density at the edge of the polishing pad is increased when compared to a pore density of the pad near the center of the wafer. This difference in pore density results in the edge of the wafer being exposed to less pad polishing surface while the center of the wafer is exposed to more polishing surface. This difference in polishing surface from the edge 14 to the center 12 may, under certain circumstances, improve chemical mechanical polishing uniformity. However, the changing of the pore density throughout a pad has some disadvantageous effects. First, the changing of the pore density of a chemical mechanical polishing pad does not reduce slurry transport problems associated with CMP non-uniformity. In fact, more pores at the edge may exacerbate this transport problem causing even further non-uniformity than before under certain processing conditions. Therefore, the uniformity improvement resulting from the use of changing pore densities of pads is self-limiting and not overly controllable.
In a second method, the integrated circuit industry has attempted to provide geometric shapes on the surface of a polishing pad to improve slurry transport. For example, geometric grooves have been formed within a top surface of the polishing pad to allow for improved uniformity of slurry transport which aids in polishing uniformity. However, it has been shown that use of grooves on polishing pad surfaces does not eliminate the non-uniformity of the polished surface and is difficult to control in a repeatable manner over pad lifetime.
In addition, the integrated circuit industry has attempted to use overhang polishing in order to render a center polish rate of a wafer and an edge polish rate of a wafer more uniform. Overhang polishing is a process whereby an edge of the wafer is moved off of the edge of a polishing pad so that a portion of the wafer is on the polishing pad while an edge portion of the wafer is not in contact with the polishing pad. In other words, an edge portion of the wafer is periodically not exposed to any polishing surface. By reducing the polish surface area exposed to the edge, the theory is that the edge polish rate will be reduced. Overhang polishing is limited since the overhang of the wafer with respect to the polishing pad can only be slight in order to avoid wafer breakage and serious wafer damage. Therefore, the uniform correcting nature of overhang polishing is inherently limited and typically results in small uniformity improvements that do not entirely eliminate the non-uniformity problem.
The integrated circuit (IC) industry has attempted to apply a backside air pressure to the center of a wafer in order to mechanically bow a wafer such that a polishing rate of a center portion of a wafer is increased with respect to the edge of the wafer. However, the backside air pressure, which is applied to the backside of a wafer, cannot exceed the mechanical pressure applied to the wafer by the wafer carrier. This inherent limitation of the use of backside air is necessary since if backside air pressure increases above the mechanical down pressure of the wafer, the wafer will no longer stay in proper position on the polishing equipment. Therefore, there is inherent limitations in backside air pressure which does not allow the industry to obtain adequate wafer polishing uniformity when backside air pressure is the only uniformity-correcting mechanism used.
Therefore, a need exists for a polishing method and/or a polishing system which will improve the center to edge uniformity of a polished semiconductor wafer and enhance the degree of controllability of the edge-to-center polish rates.